The present invention relates generally to methods and apparata for mixing and working with concrete. More particularly, the present invention relates to mixing and working with nylon or synthetic fibers which are added to concrete in order to enhance the properties of the concrete.
Anyone familiar with concrete and some of the properties as well as tendencies will know that it cracks due to settlement, shrinkage, thermal contractions and long-term drying. For years these tendencies have been accepted as a natural and foreseeable consequence of using concrete. Accordingly, contractors and masons devised ways to compensate for or minimize the magnitude of cracks in concrete as well as how soon the cracks might appear in the finished work.
One development which has shown to be quite successful is the use of small lengths of synthetic fibers such as nylon, polypropylene or polyester. While a number of companies produce such fibers and while descriptive information is similar, reference herein is made to an eight-page product brochure of Fibermesh, Inc., 4019 Industry Drive, Chattanooga, Tenn. 37416, which brochure is entitled "FIBERMESH--Engineered Synthetic Fibers for Concrete and Mortars." Various excerpts from this brochure are paraphrased below in order to provide a more detailed explanation of the synthetic fibers, their use and value.
The tendency for concrete to crack has for years been accepted as natural to its use. There is only one reason that cracks occur in concrete: stresses exist which exceed the strength of the concrete at a specific time. Stresses from external forces can be offset by providing higher structural strength to concrete structures, pavements and slabs. However, the intrinsic stresses caused by shrinkage within the concrete itself have historically been a problem to control because of their unpredictable variety and occurrence. Synthetic fibers automatically provide the dimensional stability to concrete by reducing these intrinsic stresses and relieving them until the concrete has gained sufficient strength to sustain the stresses without cracking. The elimination or reduction of early crack formation substantially reduces the number of weak planes and potential future crack formation due to thermal and drying shrinkage. (See Fibermesh brochure page 2.)
Synthetic fibers provide identical physical properties in every direction throughout the mass. Welded wire fabric is located in but a single plane and does not reduce the tendency for concrete to crack. In the hardened concrete, the presence of millions of synthetic fibers increases the ductility of the concrete; improving its impact, shatter and abrasion resistance. This reduces shattering and fragmentation under shock loading--a real asset for precast products and industrial applications. (See Fibermesh brochure page 2.)
Some fibers which are engineered exclusively for concrete use are manufactured in fibrillated form--bundles of interconnected strands. When these bundles are added to the concrete mix at the rate of 11/2 pounds per cubic yard, the mixing action causes them to open and separate into millions of individual fibers. These fibers are uniformly distributed throughout the concrete in all direcions providing effective secondary reinforcement for shrinkage crack control. As the concrete hardens, microscopic cracks develop as a result of plastic and drying shrinkage changes. When these micro-cracks intersect the nearest strand, they are blocked and prevented from developing into macro-cracks and potential problems. (See Fibermesh brochure, page 3.)
The synthetic fibers are packaged in premeasured bags ready for accurate addition to the mixing operation, either at the batching plant or into mixer trucks at the job site. The fibers are distributed throughout the concrete mixture when mixed at the mixer's rate speed and time. Overmixing will not alter its performance. (See Fibermesh brochure page 3.)
In the Fibermesh brochure, the addition of the fibers is illustrated by a photograph showing a worker on a ladder or scaffold at the top of the concrete truck drum where concrete is introduced at the batching plant. The worker has several bags of the fibers set forth in front of him and it is implied by this photograph and by this accompanying text that the bags are opened and the fibers emptied into the drum of the truck and this is in fact what occurs.
While the Fibermesh brochure refers to fibrillated bundles of fibers, other synthetic fibers may be used. For example, Nycon, Inc. of Indianapolis, Ind. produces a monofilament fiber which is believed to be particularly well suited for use with the present invention.
It would be an improvement to the widely used and accepted practice of adding fibers (of any type) if the worker could add the fibers somewhat automatically to the drum from a location at or near ground or floor level. Such an improvement would enhance the worker's safety and enable faster, easier and cleaner loading of fibers. An automated means of loading fibers would also enable more precise and accurate control so that a desired fiber weight or ratio for a particular volume of concrete could be determined and maintained somewhat uniformly, batch after batch. A means for ground level loading of the fibers precludes the need for the operator to climb any ladder or scaffolding and eliminates the need to carry the bags of fibers to the elevated location. There is also no risk that the bags of fibers might fall or split open causing either a risk to worker safety or loss.
The present invention provides the improvements outlined above as will be understood from the description which follows.